24 | New Scientist | 28 March 2020
This column appears
monthly. Up next week:
Graham LawtonChanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard modelViews Columnist
I
T IS nearly 100 years since we
confirmed that the universe –
space-time – is expanding.
But we are still struggling with
a basic fact: what is the rate of
the expansion? Depending on
how we measure a crucial number
that sets this value, we seem to
get different answers. The fallout
of this question could drastically
change our understanding of
the cosmos.
In 1929, astronomer Edwin
Hubble used observations of
galaxies to show that there was a
correlation between their velocity
and their distance from us. The
further away they were, the faster
they seemed to be receding from
our galaxy. General relativity,
which at that point had only been
around for a decade and a half, had
a clear theoretical explanation for
this finding: space-time isn’t static.
It expands, carrying galaxies
along like a raft on a river.
Hubble was able to make
this radical discovery because
of something we now call
Leavitt’s law. Discovered by
Henrietta Leavitt in 1908, the
law concerns young stars called
Cepheid variables. These stars
are called variable because their
brightness and size vary.
While working as a computer
at Harvard College Observatory,
Leavitt noticed that Cepheids had
a pattern: the power of their light
emission – absolute magnitude,
to astronomers – correlates with
the frequency of the pulsations.
In other words, by observing
these, one could calculate their
absolute magnitude, which
we could then use to calculate
how far we are from an object.
Leavitt’s law created a rung
on what we call the cosmological
distance ladder, which is a
collection of different ways that
we measure distances to objects
in the sky. Cepheids proved to bea powerful tool because they are
found in other galaxies. Hubble
took advantage of that, leading
to his finding that galaxies move
away from us at a velocity directly
proportional to their distance
from us. The proportionality
constant of this relationship is
known as the Hubble constant.
Ninety years after Leavitt
discovered her law, astronomers
were making distance
measurements using exploding
stars known as type Ia supernovae.
They found something unexpected:
not only is space-time expanding,
but that expansion is accelerating,like a river current picking
up speed. The discovery of
cosmic acceleration, as this
phenomenon is known, netted
three of the astronomers involved
a Nobel prize.
For a time, discussions of
expansion focused primarily
on the problem of explaining
cosmic acceleration. But now
astronomers are spending a lot
of time arguing over the exact
value of Hubble’s constant.
Adam Riess, one of the cosmic
acceleration Nobel laureates, has
been leading a team that used type
Ia supernova to find a value for the
Hubble constant that is 12 per cent
larger than one produced by a
different method involving the
cosmic microwave background
(CMB) radiation, the leftover
radiation from the big bang.
This difference is much larger
than the margin of error involved,
which means that the two
measurements appear to be indisagreement, assuming they are
taking all of the right physics into
account correctly.
As wonderful as Cepheids are,
because they are young stars, they
tend to be surrounded by dust.
This is a problem because it can
obscure measurements, making
them inaccurate. It means there
may be errors in Cepheid-based
calibrations of type Ia supernova
distances, leading to an error in
the distances calculated from
observations of them.
In fact, the Panchromatic
Hubble Andromeda Treasury
collaboration (PHAT) has since
found that Riess and his team
may have the wrong calibrations
for their Cepheids and this may
be partly due to their use of
ground-based telescopes as
opposed to something in space
like PHAT uses. The callibrations
also suffer from crowding – the
possibility that what looks like one
star is actually several – and this
effect can get worse with distance.
In July 2019, astronomer
Wendy Freedman said these
inconsistencies are “what keeps
[her] up at night”. She is leading
a team that is using another type
of star to measure the Hubble
constant: those in the so-called
tip of the red giant branch, known
as TRGBs. These stars are maybe
half the mass of the sun, and are
at a later stage of their lives where
helium burning has begun in their
cores, after all of their hydrogen
has burned up. Earlier this year,
Freedman’s team published a
paper saying that calibrating type
Ia supernovae with TRGBs leads to
a Hubble value that is somewhere
between that suggested by CMB
measurements and what Riess’s
team has found.
Why are these values so
different? We just don’t know –
and not knowing is part of what
makes science so fun. ❚“ The discovery of
cosmic acceleration
netted three of
the astronomers
involved a
Nobel prize”Hubble in crisis The universe is expanding, but sums at the heart
of the process don’t seem to add up. What we learn next may alter
our view of the cosmos, writes Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
I am excited to dig
into fellow scientist
Brandon Taylor’s
debut novel Real Life.What I’m watching
I am really happy
that NeNe and Porsha
made up on The Real
Housewives of Atlanta.What I’m working on
Working during a
pandemic is hard, but
I am starting some new
work involving machine
learning, and that is fun.Chanda’s week